Bicistronic CAR T Cell against BCMA and CD229 Effectively Controls Myeloma Even When BCMA Expression Is Limited

Anti-B-cell maturation antigen (BCMA) chimeric antigen receptor (CAR) T-cell therapy has revolutionized the prognosis of patients with relapsed/refractory multiple myeloma. Regrettably, despite unprecedented overall response rates achieved with this approach, most patients eventually relapse. One of the primary reasons for this is the complete loss or reduced expression of BCMA on the malignant plasma cell surface. Consequently, new therapeutic targets are under investigation. Another potential therapeutic approach involves the use of CAR T cells targeting two tumor antigens. In this study, we developed and validated a monospecific CAR targeting CD229, which was effective in in vitro and in vivo NOD.Cg-Prkdcscid Il2rdtm1Wjl/SzJ mouse models with both homogeneous and heterogeneous BCMA expression. Additionally, we created a bicistronic CAR T cell targeting both CD229 and BCMA, which demonstrated efficacy in models with homogeneous BCMA expression, in heterogeneous models featuring small clonal populations with biallelic BCMA deletion, and in cases with reduced BCMA expression both in vivo and in vitro. Regarding "on-target off-tumor toxicity," no fratricide was observed among CAR T cells, but there was a limited elimination of nonactivated T cells. The immune pressure exerted by anti-CD229 CAR T cells led to the loss of the CD229 antigen expression in some instances. In summary, this work underscores the potential utility of CD229 alone or in combination with BCMA as an immunotherapeutic target in multiple myeloma, especially in cases marked by diminished or absent BCMA expression.